1. Field of the Invention
The present invention relates generally to a storage apparatus in which when reading a zone constant angular velocity (ZCAV) recorded medium, a read clock frequency is variably controlled in response to zones of the medium, and more particularly to a storage apparatus having a frequency synthesizer in which are set frequency dividing ratios corresponding to the medium zones, to optimize a wait control of settling time required for the varied frequency to become stabled.
2. Description of the Related Art
Magneto-optic disk media known as MO cartridges commercially available at present include 128 MB, 230 MB, 540 MB and 640 MB media and direct overwritable 230 MB, 540 MB and 640 MB media. Among them, the 230 MB, 540 MB and 640 MB media employ the ZCAV recording while the 128 MB medium having a single zone employs mere CAV recording.
For the ZCAV recording, a medium data area is radially divided into a plurality of zones, with the outer zones having higher recording frequencies. When reading data from a ZCAV recorded medium by means of an optical disk drive, it is therefore necessary to vary a reference clock frequency depending on a target zone. Setting of this reference clock is usually effected by a frequency synthesizer using a PLL within a data read only LSI called a read LSI. More specifically, an externally generated clock of a frequency fi is fed to the frequency synthesizer, in which a frequency dividing ratio (m/n) is set using two digital values m and n received from an MPU, to output a reference clock having a frequency fo given as EQU fo=(m/n) fi
When reading zone-to-zone data, it is further necessary in addition to the setting of the frequency synthesizer to set other parameters such as a filter cut-off frequency. Such setting is also effected by writing digital values into the read LSI.
In such a conventional optical disk drive, however, when the MPU sets a frequency dividing ratio in the frequency synthesizer of the read LSI to thereby vary the frequency, time is required for a reference clock to become stabilized at a target frequency after writing the frequency dividing value into the read LSI. This time is usually called settling time. The settling time is proportional to a variation in the frequency dividing ratio to be set. An execution of read processing with an unstable reference clock frequency generated from the frequency synthesizer may bring about an error, so that any processing must be stood by until the reference clock becomes settled. Up till now, in order to determine this settling wait time, the frequency dividing ratio of the frequency synthesizer is varied to its maximum within a frequency range necessary for the medium, to actually measure the settling time at that time. The thus measured maximum settling time is uniformly set as a wait time after the setting of the frequency dividing ratio in the frequency synthesizer. For this reason, even in the case of less amount of zone-to-zone movement, the maximum settling time corresponding to the medium frequency range is employed as the wait time, resulting in an vain wait in spite of actual settling, so that data transfer to the host apparatus may be apparently decelerated.